This study was designed to examine the effects of short photoperiod (SP) exposure on gonadotropin and PRL levels in the presence and absence of estrogen treatment in ovariectomized LSH/SsLak hamsters. In experiment I, regularly cycling hamsters maintained in long photoperiod (14L:10D) were ovariectomized and Silastic capsules containing 2.0-mm columns of estradiol benzoate (EB) implanted simultaneously into half of the animals. On the following day, half of the animals in each treatment group were transferred to SP (8L:16D). After 20 days of SP or long photoperiod (LP) exposure, all animals were sacrificed by decapitation and their sera and pituitaries saved for hormonal determinations. The experimental protocol in experiment II was similar, except that two groups in each photoperiod received estrogen treatments; one group received 2.0-mm implants of 17-beta-estradiol (E2), whereas a second group received 10.0-mm E2 implants. SP treatment effected a reduction in serum LH and FSH levels in the absence of steroid replacement treatment. EB treatment depressed serum gonadotropin levels in LP animals, but did not alter levels in SP hamsters. In experiment II, LP- and SP-treated animals showed similar responses to E2 treatment, although different responses were noted in the two dosage groups. Pituitary gonadotropin contents became progressively decreased with increasing steroid levels and in certain groups showed SP-induced reductions. Serum and pituitary PRL levels increased in response to steroid treatment, but were not affected by SP treatment. In summary, 20 days of SP treatment caused gonad-independent reductions in gonadotropin levels and appeared to reduce the steroid feedback sensitivity of the hypothalamo-pituitary axis. PRL levels were not affected by SP exposure in the absence of the ovaries, and the interaction of PRL and estrogen appeared to be similar in both photoperiods.